The present invention relates to printed circuit boards and a method for manufacturing such printed circuit boards.
Printed circuit boards such as flexible printed circuit boards are widely used in a variety of electrical and electronic devices. A printed circuit board includes, for example, an insulating layer of, e.g., polyimide, having on one side or both sides thereof conductor layers of, e.g., a copper foil with prescribed patterns.
Conventionally, conductor layers having prescribed patterns in printed circuit boards such as flexible printed circuit boards are formed by the known methods such as the semi-additive, substractive, and full-additive methods (refer to JP 2002-176259, for example).
The above-mentioned semiadditive method is now described with reference to the drawings.
FIGS. 4 (a), 4 (b), 4 (c), 4 (d), 4 (e), and 4 (f) are schematic cross sectional views showing the steps of a method for manufacturing a printed circuit board by the semiadditive method.
In the semi-additive method, for example, an insulating layer 11 of a resin film is first prepared, as shown in FIG. 4 (a).
Next, as shown in FIG. 4 (b), a thin conductive film 12 is formed on the insulating layer 11 by sputtering or electroless plating.
Then, as shown in FIG. 4 (c), a plating resist 13 is formed on the thin conductive film 12 using a dry film resist or the like. The plating resist 13 has patterns opposite to the prescribed patterns of a conductor layer formed in a step shown below.
After this, as shown in FIG. 4 (d), a conductor layer 14 is formed by electrolytic plating on the surfaces of the thin conductive film 12 where the plating resist 13 is not formed.
This is followed by removing the plating resist 13 by, e.g., stripping, as shown in FIG. 4 (e). Then, the thin conductive film 12 except the portions on which the conductor layer 14 is formed is removed by chemical etching or the like, as shown in FIG. 4 (f). In this manner, the conductor layer 14 with prescribed patters is formed on the insulating layer 11.
However, in the conventional method for forming a printed circuit board, side etching of the thin conductive film 12 under the conductor layer 14 may occur during the removal of the thin conductive film 12 except the portions under the conductor layer 14 by chemical etching or the like. The term “side etching” is described now with reference to the figure below.
FIG. 5 is a magnified view of the region B in FIG. 4 (f). During the removal of the thin conductive film 12 by chemical etching or the like, the thin conductive film 12 under the conductor layer 14 was etched together with both ends thereof being gouged as shown in FIG. 5. This results in a deterioration of the adhesion of the conductor layer 14 to the thin conductive film 12. In the case of a significant decrease in the adhesion, the conductor layer 14 may be stripped off.